The shift in military posture would allow North Korea to carry out pre-emptive attacks, and deprive "the enemies to sleep in peace till the moment they meet their final end in their land," KCNA reported, citing a speech by the country's leader, Kim Jong-un.

KCNA continued: "The only way for defending the sovereignty of our nation and its right to existence under the present extreme situation is to bolster up nuclear force both in quality and quantity and keep balance of forces," he said, stressing the need to "get the nuclear warheads deployed for national defense always on standby so as to be fired any moment."

The move follows the U.N. Security Council's unanimous approval Wednesday of tough new sanctions against North Korea in response to its recent nuclear and long-range missile tests. The resolution contains the toughest set of sanctions imposed by the Security Council in more than two decades, Secretary of State John Kerry said.

The sanctions require North Korean cargo ships and aircraft to be inspected before entering and after leaving the reclusive country. They would also prohibit small arms and other conventional weapons sales to North Korea.

The Pentagon said it is aware of the reports and closely monitoring the situation in coordination with regional allies.

Gabrielle Price, the spokeswoman for the State Department's Bureau of East Asian and Pacific Affairs, said: "We urge North Korea to refrain from provocative actions and rhetoric that aggravate tensions and instead focus on fulfilling its international obligations and commitments."

Kim Jong Un(Photo: KNS, AFP/Getty Images)

KCNA, in its typically dramatic language, called the sanctions a flagrant effort by the "U.S. imperialists and their followers" to impose "gangster-like" economic and political pressure and military aggression on North Korea for its defensive actions and sovereign right to launch an earth-observation satellite.

"All the people in the DPRK are now waiting for an order of combat to annihilate the enemy with their surging wrath at the U.S. imperialists and south Korea's Park Geun Hye group of traitors," the agency said, referring to the South's president.

Just hours after the Security Council resolution passed, North Korea fired several short-range projectiles into the sea, according to the South Korean defense ministry.

The extent of North Korea's nuclear arsenal is uncertain. The nation claimed in January that it had successfully detonated a hydrogen bomb at a test site, but third-party experts and U.S. leaders expressed doubts. Chinese officials told U.S. nuclear specialists in April that North Korea may possess 20 nuclear weapons and produce 10 new bombs a year, according to a report in The Wall Street Journal.

Bruce Klingner, a former head of the CIA's Korea branch now at the Heritage Foundation, said the best estimates are that the North has 10-16 nukes, though it is uncertain whether they are small enough to put on missiles.

Klingner believes North Korea's Nodong missile is nuclear capable, meaning Japan and South Korea are in range, together with the 28,500 American military personnel stationed in the South and the 45,000 U.S. military personnel stationed in Japan.

Three U.S. generals in charge of U.S. forces in Korea and the Pacific and North American Aerospace Defense Command (NORAD) testified last year that North Korea has the capability to hit the continental United States with a nuclear warhead. The missile that launched a North Korean satellite into orbit Feb. 7 had a range of 8,000 miles, putting all of the West Coast in Kim Jong-un's sights, Klingner said.

While North Korea has not demonstrated the ability to bring a payload back into earth's atmosphere without burning up, the nation's capabilities are dangerous and the risk of miscalculation is increasing, he said.

The United States is preparing for yearly military exercises on the Korean peninsula in March and April, and it recently announced new war plans with South Korea that include possible preemptive attacks and decapitation of the North Korean regime.

The United States is also building a network of ground-based missile interceptors designed to counter a threat from countries such as North Korea. The Pentagon said it is on track to have 44 such interceptors ready by the end of next year. Some are already on line.

It's unclear if North Korea's leadership will miscalculate these changes and the coming exercises as preparations for an attack, Klingner said.

North Korea's announcement "raises tensions," Klingner said. "When you have someone threaten use of nuclear weapons you can't help but have concerns."